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Sigtronics ICS Load Modification Instructions Intercom Models: Sport 200 SPA-400 SPA-600 SDB-800 ST-400 ST-600 This modification is used when a Sigtronics Intercom is connected to a radio or audio panel which has a low impedance headphone output (less than 100 ohms). This is most common with some Narco equipment (CP-135,CP-136, and MARK 12D for example) and “home made” audio select panels. Symptoms Are: Intercom audio volume is weak or non-existent. Transmit function is normal on VHF radio. Receive function is normal on VHF radio. Verification: Disconnect the intercom headphone wire (blue) from the radio or audio panel. Make sure that the blue wire is still connected between the intercom and all intercom headphone jacks. If the maximum intercom volume level is now approximately 30% or more greater than when the blue wire was connected, then this modification will solve the problem. Solution: Add a 1/4 or 1/2 watt resistor per the drawing below. The value of the resistor most commonly used is 220 ohms, however, optimum performance can be achieved by selecting the value right for your particular installation. The resistor can be any value between 100 and 330 ohms and is selected for the best balance between radio receive volume and intercom volume. The example below is for a Sport 200 intercom. The resistor modification applies to all listed intercoms. AIRCRAFT / INTERCOM INTERFACE AREA TO AIRCRAFT POWER BUSS 12 /28 VDC HEADPHONE AUDIO TIP POINT AIRCRAFT HEADPHONE JACK ( INTERCOM CENTRAL GROUND POINT ) (7) RED (4) BLACK INTERCOM CIRCUIT BREAKER ( 1 AMP ) TIP RING BARREL SQ ON J1 TIP AIRCRAFT HAND MIC JACK TO PILOT PTT SW. TO PILOT MIC JACK (1) WHITE / BLACK VOL Sport 200 RING 12345 RADIO MIC KEY BARREL (3) BLUE P1 TIP BREAK CONNECTION AND ADD RESISTOR IN-LINE HERE. PILOT HEADPHONE JACK AIRCRAFT RADIO MIC AUDIO CO-PILOT PTT SW. TO (6) BROWN INTERCOM Sport 200 INTERCOM (5) WHITE (2) WHITE / RED (10) WHITE / BLUE J1 (11) WHITE / ORANGE 2 (1 IO )V T LE A TO A TO CO-PILOT HEADPHONE JACK BARREL TIP RING TIP TO BARREL CO-PILOT MIC JACK NOTES 1. IF PILOT PTT SW IS CONNECTED TO THE AIRCRAFT HAND MIC JACK, IT MUST BE DISCONNECTED AND CONNECTED TO THE WHITE / RED WIRE AS SHOWN. 2. � AIRCRAFT CHASSIS GROUND. Optional Push-To-Intercom Switches ( 1 Per Headset ) Specialists in “SOUND” Management 178 East Arrow Highway, San Dimas, CA 91773 ( 909 ) 305-9399 1-12-2007 ICS-Load_1440_Mod.pdf